For example, I have a pal who is working on porting the bitcoin client to Solaris on an Ultrasparc3 Sunfire. I guess he would be affronted by being pushed into the Linux, BSD and other category
There is some flexibility in the interpretation of "or similar" for the Linux or BSD option because I recognize there are varying degrees of similarity between Unix-like systems. I would say if bitcoin compiles and works with little or no tweaks, then it is "similar" for the purposes of the poll.
If your friend cannot easily compile, and is having to port to Solaris, then I would say that would belong in the 'Other' category, not because it's in any way inferior or not Unix-like, but simply because it's not similar enough to a supported configuration.